The present invention relates generally to a super-conductive coil in a synchronous rotating machine. More particularly, the present invention relates to a rotor having a super-conductive core, coil support structures and an electromagnetic shell.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core super-conductive rotors require large amounts of super-conducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor.
High temperature SC coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K, to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
Super-conducting coils have been cooled by liquid helium. After passing through the windings of the rotor, the hot, used helium is returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power.
Prior SC coil cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a porous SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. However, immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature, as a result no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures.
What is needed is a super-conducting field winding assemblage for an electrical machine that does not have the disadvantages of the air-core and liquid-cooled super-conducting field winding assemblages of, for example, known super-conductive rotors.
In addition, high temperature super-conducting (HTS) coils are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start up and shut down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor winding should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature and notwithstanding occasional over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the windings over normal operating conditions.
High strains can damage HTS super-conductor wire. To withstand high strains, HTS wire have in the past been protected by massive and complex coil winding and coil support structures. However, massive, complex super-conducting windings and supports are costly, especially in state-of-the-art air core electrical machines. Moreover, these massive windings have to be cooled to cryogenic temperatures, and thus require large refrigeration systems.
The coil windings also are isolated from the hot coil supports and rotor. To isolate the coil windings, large thermal insulators have been used to separate the coils from their support systems. Because the insulators are between the coils and their support systems, prior thermal insulators are large structures that can support the high centrifugal loading of coils. Because these large thermal insulators are in contact with the cold coils, the insulators are a large heat source to the coils. While the isolators are designed to minimize heat conduction to the coils, the insulators result in large cryogenic heat loads and expensive cryorefrigerators.
Developing support systems for HTS coil has been a difficult challenge in adapting SC coils to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a HTS rotor having a coil support system for a SC coil. The need also exists for a coil support system made with low cost and easy to fabricate components.
A high temperature super-conducting (HTS) rotor has been developed having a two-pole core body formed of a solid magnetic material, such as iron. The rotor core body is generally cylindrical and has flat surfaces machined longitudinally along its length. The HTS coil is assembled around these flat surfaces and the coil has a race-track shape that extends around the core. The race-track coil is supported by tension coil support members that extend through the iron core rotor body. Drive and collector shafts are mechanically fastened to the rotor core. A cylindrical shell electromagnetic shield surrounds the HTS coil and iron core rotor body.
The iron core rotor significantly reduces the field winding ampere-turns, super-conductor utilization and cost with respect to air-cooled rotors. The single race-track shaped HTS coil replaces typical complex saddle-shaped coil windings. The tension coil support provides direct support to the HTS coil so as to reduce the strains on the coil during cool-down and centrifugal loading. Moreover, the coil support system is at cryogenic temperatures with the coil.
The HTS rotor may be implemented in a machine originally designed to include a SC coil(s). Alternatively, the HTS rotor may be implemented to replace a conventional rotor field winding of a two-pole synchronous machine with a single race-track shaped high temperature super-conducting (HTS) coil. The rotor and its SC coil are described in the context of a generator, but the HTS coil rotor and coil support disclosed here are also suitable for use in other synchronous machines.
In a first embodiment the invention is a rotor for a synchronous machine comprising: a cylindrical magnetic solid rotor core; a race-track super-conducting coil winding extending around the rotor core; a coil support extending through the core and attaching to opposite long sides of the coil winding, and a pair of end shafts extending axially from said core and attached to the core.
In a second embodiment of the invention is a method for assembling a high temperature super-conducting rotor having a coil winding on a solid iron rotor core of a synchronous machine comprising the steps of: extending a tension bar through a conduit in said rotor core, wherein said conduit extends between opposite planer sections on long sides of the core; inserting a housing over a portion of the coil; attaching an end of the tension bar to the housing, and attaching rotor end shafts to opposite ends of the rotor core.
In a further embodiment the invention is a rotor in a synchronous machine comprising: a cylindrical rotor core having a pair of planer sections on opposite sides of the core and extending longitudinally along the core; a super-conducting coil winding extending around at least a portion of the rotor core, the coil winding having a pair of side sections adjacent the planer sections of the core; a first end shaft extending axially from a first end of the rotor core, and a second end shaft extending axially from a second end of the rotor core.